The present invention relates to an optical memory medium for optically recording and reproducing information which is especially suitable to the information recording and erasing and also relates to an information recording and erasing method and apparatus of an optical memory medium which are suitable for surely recording and erasing information of the optical memory medium at high speed.
An apparatus for recording information at high density and reproducing it by using laser light condensed to a submicron order is capable of recording a large capacity of information such as video information or audio information and is expected to become an indispensable apparatus for the future information society. One of such recording and reproducing apparatuses is a known optical disk apparatus. In optical disk apparatuses, there are three types: (1) reproduction-only type; (2) postscript type; and (3) rewritable type. The reproduction-only type and the overwriting type have nearly reached the stage of practical use. On the other hand, no methods are established in the rewritable type yet. The rewritable optical disk using an optical magnetic material, phase change material or the like is now under vigorous research and development. In the opto-magnetic disks and optical disks using the phase change material now under research, information is erased by using laser light. In case of optical magnetic disks, however, the magnetic field must be inverted during the erasing operation. Accordingly, optical magnetic disks are not suitable to the so-called successive erasing and recording scheme in which the information recording is effected concurrently with the information erasing. In case of an optical disk using the phase change, the recording sensitivity is lower than the erasing sensitivity and hence a laser beam taking the shape of long ellipse is radiated for the purpose of erasing operation.
The optical disk using the phase change comprises at least a substrate, a recording material layer and a guard film layer. The recording material used for the optical disk is a material highly sensitive to electromagnetic waves such as laser light. That is to say, the recording material used for the optical disk is a recording material using the crystal-amorphous phase change represented by a recording material disclosed in U.S. Pat. No. 3341825 or an organic memory material using a pigment or polymer as disclosed in JP-A-60-166487, JP-A-60-15180 and JP-A-60-180887. The features of these material are as follows: (1) Although recording operation can be effected at high speed, erasing operation is slow; (2) The erasing temperature is higher or slightly lower than the heat-resisting temperature of the substrate (120.degree. to 150.degree. C. in case of a plastic substrate). If these recording materials are used for the erasing operation, therefore, a technique for heating the recording material for a long time without heating the substrate is required.
If a long ellipse laser beam provided apart from the laser beam for recording and reproducing operation is radiated to erase the information in accordance with the conventional method, the ratio of the major axis to the minor axis is limited to approximately 10:1. The material whose data can be erased by using an ellipse beam having the axis ratio close to 10:1 is poor in thermal stability and hence in long-term stability of data. In case of a material having a fine thermal stability, the data cannot be erased by an ellipse beam having an axis ratio of approximately 10:1. Further, high-speed erasing operation is difficult.
In addition, the erasing temperature is higher than or close to the heat-resistance temperature of the substrate. In spite of the fact that the data can be erased by heating the material for a long time, therefore, the data cannot be actually erased for preventing the substrate from being heated.